Durable broad-spectrum, adult-plant stem rust resistance in wheat conferred by the Sr2 gene has remained effective against Puccinia graminis f. sp tritici worldwide for more than 50 years. The Sr2 gene has been positioned on the physical map of wheat to the distal 25% portion of the short arm of chromosome 3B. Selection for this gene in wheat breeding programs within Australia has been performed so far through the use of the linked pseudo black chaff (PBC) phenotype and of the microsatellite markers Xgwm389 and Xgwm533 that flank the gene. The molecular markers flank a genetic interval of approximately 4 cM equating to a physical distance of over 10 Mbp. Recently, a 3B-specific BAC library was developed and a physical map established for this region. Analysis of the sequence of minimal tiling path-BAC clones within the region containing the Sr2 gene enabled the development of three new markers that were mapped within the Xgwm389-Xgwm533 genetic interval and tightly linked to the Sr2 gene. Screening a wide range of germplasm containing the Sr2 gene with these markers demonstrated their usefulness for marker-assisted selection in Australian wheat breeding programs.
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The stem rust resistance gene Sr2 has provided broad-spectrum protection against stem rust (Puccinia graminis Pers. f. sp. tritici) since its wide spread deployment in wheat from the 1940s. Because Sr2 confers partial resistance which is difficult to select under field conditions, a DNA marker is desirable that accurately predicts Sr2 in diverse wheat germplasm. Using DNA sequence derived from the vicinity of the Sr2 locus, we developed a cleaved amplified polymorphic sequence (CAPS) marker that is associated with the presence or absence of the gene in 115 of 122 (95%) diverse wheat lines. The marker genotype predicted the absence of the gene in 100% of lines which were considered to lack Sr2. Discrepancies were observed in lines that were predicted to carry Sr2 but failed to show the CAPS marker. Given the high level of accuracy observed, the marker provides breeders with a selection tool for one of the most important disease resistance genes of wheat.
Sr2 is the only known durable, race non-specific adult plant stem rust resistance gene in wheat. The Sr2 gene was shown to be tightly linked to the leaf rust resistance gene Lr27 and to powdery mildew resistance. An analysis of recombinants and mutants suggests that a single gene on chromosome arm 3BS may be responsible for resistance to these three fungal pathogens. The resistance functions of the Sr2 locus are compared and contrasted with those of the adult plant resistance gene Lr34.
The recent emergence of wheat stem rust Ug99 and evolution of new races within the lineage threatens global wheat production because they overcome widely deployed stem rust resistance (Sr) genes that had been effective for many years. To identify loci conferring adult plant resistance to races of Ug99 in wheat, we employed an association mapping approach for 276 current spring wheat breeding lines from the International Maize and Wheat Improvement Center (CIMMYT). Breeding lines were genotyped with Diversity Array Technology (DArT) and microsatellite markers. Phenotypic data was collected on these lines for stem rust race Ug99 resistance at the adult plant stage in the stem rust resistance screening nursery in Njoro, Kenya in seasons 2008, 2009 and 2010. Fifteen marker loci were found to be significantly associated with stem rust resistance. Several markers appeared to be linked to known Sr genes, while other significant markers were located in chromosome regions where no Sr genes have been previously reported. Most of these new loci colocalized with QTLs identified recently in different biparental populations. Using the same data and Q + K covariate matrices, we investigated the interactions among marker loci using linear regression models to calculate P values for pairwise marker interactions. Resistance marker loci including the Sr2 locus on 3BS and the wPt1859 locus on 7DL had significant interaction effects with other loci in the same chromosome arm and with markers on chromosome 6B. Other resistance marker loci had significant pairwise interactions with markers on different chromosomes. Based on these results, we propose that a complex network of gene-gene interactions is, in part, responsible for resistance to Ug99. Further investigation may provide insight for understanding mechanisms that contribute to this resistance gene network.
Stem rust resistance in wheat (Triticum aestivum L.) conferred by the Sr2 gene has remained effective against Puccinia graminis Pers.:Pers. U sp. tritici Eriks. & E. Henn. worldwide for more than 50 yr. Sr2 resistance is associated with variable levels of disease symptoms; it shows recessive inheritance and is expressed primarily during the adult-plant stage. A genetic and physical interval for the Sr2 gene was determined on the short arm of chromosome 3B of wheat. Flanking microsatellite markers were identified and one tightly linked marker (gwm533) was shown to be associated with presence of Sr2. This marker was validated in a wide range of germplasm and can now be implemented in marker-assisted breeding to facilitate selection for this durable, broad-spectrum but difficult to score rust resistance gene.